High-capacity, high-endurance batteries have been used for many years in locomotives, submarines and electric power stations to name only a few examples. These days, high-reliability batteries are becoming more popular in electric cars and electric buses. These batteries represent a large investment for a user and are expected to last a long time.
These batteries are subject to high amperage during peak demand. They are subject to shock and vibration when used in road vehicles, and to expansion and contraction by heat and cold. For these reasons, the electrical resistance and the structural integrity of any connection inside a battery are directly related to the performance and longevity of that battery.
For reference purposes, examples of electrochemical batteries of the prior art are described below.
U.S. Pat. No. 1,377,087 issued to J. A. Manns on May 3, 1921; and U.S. Pat. No. 6,308,639 issued to F. W. Donnelly et al., on Oct. 30, 2001, illustrate well the magnitude of battery sizes and power used in electric locomotives.
US Patent Application US 2001/0036574 published on Nov. 1, 2011, by S. Fukuda et al.; and
US Patent Application US 2004/0229121 published on Nov. 18, 2004, by S. Hamada et al., are related to the structure of electrode plates in electrochemical batteries. Both documents teach a structure and a method for manufacturing electrode plates for use in electrochemical batteries.
More specifically, the latter documents describe rechargeable batteries wherein each electrode plate is connected to a lead bar and to a bus bar by electronic welding, seam welding or resistance welding.
It will be appreciated that the conductance of an electrode plate is the primary concern herein. As mentioned before, the welding of a joint on that electrode plate has detrimental effects on the efficiency and durability of the battery in which the electrode plate is mounted.
The best prior art found teach batteries having brazed connections, and joints made by resistance welding. Firstly, by definition, a brazed connection contains at least two materials in contact with each other; a base metal and a filler metal. The filler metal must have a significantly lower melting temperature than the base metal of the pieces to be joined so that the filler metal may properly wet the pieces to be joined. When heated, the filler metal spreads in the gap of the joint by capillary action, but creates no metallurgical bond with the base metal.
Because of the filler metal in a brazed connection, the connection has different electrical conductivities and different mechanical properties at the interface.
Because of the gap between the parts to be joined, there are uneven thicknesses of the filler metal in the gap. The electrical resistance of the connection differs from that of the base metal.
Because of the filler metal, a brazed connection has thermal mismatches (different thermal expansion coefficients) causing thermal stresses (hot spots) at the interface and thermal fatigue at joints due to repetitive heating/cooling. This leads to cracking of the connection in time.
A brazed connection in a battery has adverse effects on the performance and durability of the battery because it creates voltage drop and cracking.
Secondly, a joint made by resistance welding uses high current and pressure to cause localized fusion of the parts to be joined. This process is predominantly used for spot welding.
There are impurities between the surfaces to be joined when these surfaces are melted together. Undeniably there are dust particles in all shop environments. These dust particles find their ways into any manufacturing process.
The surfaces of both parts are coming from two different foundry stocks and do not have identical metallurgical contents. These parts do not have identical thicknesses, and therefore not the same resistance to the welding current. The surfaces of the mating parts have been affected to various degrees by oxidation, simply by being exposed to a shop environment.
And yet, even a surface with zero roughness contains microscopic mounds and valleys entrapping air pockets and water vapours and other gases found in a manufacturing shop environment. The resulting weld contains porosity and microscopic laminations.
In a resistance welding process, it is almost impossible to evenly distribute the flow of electron across the surface of the joint. Because of this difficulty, a joint made by resistance welding contains, at a microscopic level, a certain amount of metal being burnt, a certain amount of metal that is molten according to ideal conditions, and a certain amount of metal that has not been sufficiently molten. And in all cases, the tempering of the metal is lost, often causing brittleness around the joint.
Because of the burnt metal; brittleness; oxidation; porosity and laminations, a joint made by resistance welding also has less than ideal conductance, and thermal mismatches leading to thermal stresses and cracking of the connection in time.
Therefore, both connections made by resistance welding and by brazing inside a battery have adverse effects on the performance and durability of the battery because of the resulting voltage drop and cracking.
For these reasons, basically, there is a long felt need in the field of high-endurance, high-capacity batteries for an electrode plate that has better joints than those described in the prior art literature.